1. Field of the Invention
The present invention relates tires and methods for making tires. More specifically, the present invention concerns a tire and a method for making a tire having a particular belt structure.
2. Description of the Related Art
As is known, a tire includes a carcass, a tread band, a pair of sidewalls and reinforcing layers between the tread band and the carcass. The carcass, usually at least a single ply, is turned out at its ends around a pair of bead cores. The bead cores, the ends of the carcass, and whatever filler that may be added between the bead cores and carcass form the beads on either side of the tire.
In the traditional (or known) tire, there are generally at least three rubber layers below the tread band, two belted layers and a 0 degree layer or nylon layer. The nylon layer contains reinforcing cords, preferably of a textile material, and more preferably of a heat-shrinkable material, such as nylon, oriented at 0xc2x0 (i.e., in a circumferential direction) with respect to the equatorial plane of the tire. The reinforcing cords preferably are disposed in a sheet made of elastomeric compound. The nylon layer is located between the tread band and the belted layers. As understood by those skilled in the art, the nylon layer stabilizes the tire, improves its rolling resistance, and generally enhances its operational characteristics because the nylon layer helps the tire retain its shape when subjected to large centrifugal forces at high speeds. Without the nylon layer, the performance of the tire may be compromised, especially at high speeds.
The two belted layers comprise fine cords, preferably metallic cords, displaced in an elastomeric compound, that crisscross each other and are both angled with respect to the equatorial plane of the tire.
In addition to the three reinforcing layers, the traditional tire may also include a rubber sheet between the nylon layer and the tread band, which must be added because of the manner in which traditional tires are manufactured. As is known, the tread band is, often, a separately manufactured, extruded product. Because it is manufactured separately, the tread band cools to room temperature before it is incorporated into a tire. Upon cooling, the tread band loses tackiness and is less able to stick properly to the underlying layer during construction of the tire. To compensate for the loss of tackiness and to improve the adherence of the tread band to the underlying layer so that the tire may be assembled properly before it is vulcanized, a rubber sheet is added to the underside of the tread band. The rubber sheet is a calendered or coestruded (with the tread band) sheet of an elastomeric compound based on natural rubber that is well known in the art. The rubber sheet is usually 0.2 to 1.0 mm thick. Its tackiness may be attributed to the fact that the rubber sheet is typically made from a compound with a natural rubber content of more than about 50 percent of the polymer amount.
An alternate construction of the traditional tire, known as xe2x80x9ccap and base tyrexe2x80x9d, further includes an underlayer as part of the tread band on the side of the tread band facing the interior of the tire. The underlayer improves handling while reducing the rolling resistance of the tire and is incorporated into the tread band between the tread band and the underlying layer. In this alternate construction, the various layers of the tires are arranged in the following manner from the exterior of the tire to the interior: (1) the tread band, (2) the underlayer, which is incorporated into the underside of the tread band, (3) the rubber sheet, and (4) the 0xc2x0 nylon layer, which is placed above the belted plies. The underlayer is made from a rubber-based compound and is usually between 1 and 2 mm thick. Since it does not have the same composition and characteristics as the tread band, the thickness of the underlayer is selected so that it will not come into contact with the road when the tire tread wears out.
In situations where the underlayer is made, for example, from a compound with a natural rubber content greater than 50 percent of the polymer amount, the rubber sheet may be omitted because the underlayer will have sufficient tackiness to properly adhere the tread band to the carcass, as described above in relation to the rubber sheet.
This does not mean, however, that the rubber sheet is omitted in every case where the natural rubber content of the overlaying layer is greater than 50%. In some cases, the rubber sheet can be omitted despite the fact that the rubber content of the overlaying layer is less than 50%, for example in the case that other material, able to make the overlaying layer with a good tackiness, are added to the compound, or for example when the tread band is not left to cool.
It is also known in the art to construct elastomeric compounds for tires that are reinforced through the addition of reinforcing fibers. One such example is U.S. Pat. No. 4,871,004 (xe2x80x9cthe ""004 patentxe2x80x9d), which describes the addition of Kevlar(copyright) pulp (a registered trademark of DuPont) to tire rubber to a point where the Kevlar(copyright) has a concentration of 0.2 to 20 parts per hundred of rubber (phr). In the ""004 patent, the Kevlar(copyright) fibers are described as having a length (L) of 0.2 to 5 mm, a diameter (D) of 0.005 to 0.02 mm, and an aspect ratio (L/D) of greater than about 100. The resulting mixture of fibers and rubber can be used to prepare any one of the described components of a tire: apexes, filler strips, belts, belt overlay, and gum strips. The ""004 patent describes some of the advantages of adding reinforcing fibers to an elastomer in the construction of a pneumatic tire, such as its reinforcing effect and the increased stiffness of the resulting tire.
European Patent Number 0 592 218 A1 (xe2x80x9cthe EPO ""218 patentxe2x80x9d) also describes some of the advantages of adding reinforcing fibers to the rubber for different parts of a pneumatic tire. The EPO ""218 patent describes the addition of 16 to 30 parts per hundred of rubber (phr) of short fibers having an average diameter (D) of 0.1 to 0.5 xcexcm, an average length (L) of 40 to 500 xcexcm, and an aspect ratio (L/D) of 100 to 5000. Examples of the fibers that can be added are aramid fibers such as Kevlar(copyright), cotton, nylon, polyester, rayon, and surface treated short fibers such as Nylon 6.
Similarly, European Patent Number 0 604 108 A1 (xe2x80x9cthe EPO ""108 patentxe2x80x9d) describes the addition of certain types of fibers to reinforce the rubber in a pneumatic tire. The specific fibers discussed by the EPO ""108 patent are short fibers of UBEPOL-HE 0100 (available from UBE Industries Limited), i.e., Nylon-6. The fibers are described as having an average diameter (D) of 0.3 xcexcm, an average length (L) of 300 xcexcm, and an aspect ratio (L/D) of 1000. The fibers are incorporated into the rubber compound in a concentration of not less than 10 parts per 100 of rubber (phr), and preferably 10 to 30 phr.
The present invention concerns the construction of an elastomeric intermediate layer between the tread band and the belted plies of a pneumatic tire that replaces the nylon layer of the traditional tire but retains the beneficial characteristics of such structural element. Further, the intermediate layer may replace the rubber sheet if present. Similarly, for the second example of the traditional tire, the present invention provides a construction where the elastomeric intermediate layer replaces the 0xc2x0 nylon layer, the underlayer, and the rubber sheet (if present), to achieve the same goals.
The Applicant found that replacing one or more layers in the traditional tire by introducing an elastomeric intermediate layer to the tread band made with a fiber-reinforcing material, the present invention realizes at least the same advantages in rolling resistance and handling that are previous realized by the structures of both of the traditional tire designs but it does so while reducing the overall weight of the tire.
Moreover, the replacement of the traditional structures with fewer layers, simplify the construction of a tire.
The present invention contemplates adding short fibers such as Kevlar(copyright) (in the form of Kevlar(copyright) pulp) to the elastomeric intermediate layer preferably incorporated into the tread band to compensate for the elimination of the traditional 0xc2x0 nylon layer, rubber sheet (if present), and underlayer (if present) from the construction of the traditional tire designs. The present invention contemplates the addition of short fibers having a trunk portion with a diameter of about 10 microns, a length of about 200 microns, and an aspect ratio of about 20. Together with other features of the present invention, the addition of short fibers (such as Kevlar(copyright)) helps to keep the benefits of rolling resistance and handling offered by the two traditional tire designs. Similarly, Twaron(copyright) pulp (a registered trademark of Akzo Nobel), another type of pulp including a reinforcing fiber, may be used in the place of Kevlar(copyright) pulp. Due to the addition of fibers, the elastomeric intermediate layer also is referred to as a xe2x80x9cfeltedxe2x80x9d layer.
The present invention further recognizes that the elastomeric intermediate layer is made from the combination of the short fibers with a compound material. The compound material may comprise natural rubber, emulsion-polymerized styrene butadiene rubber, solution polymerized styrene butadiene rubber, and butadiene rubber. Preferably, the compound material is a combination of about 60% natural rubber with about 40% emulsion-polymerized styrene butadiene rubber. The present invention contemplates adding the short reinforcing fibers to the compound material in a concentration between about 5 and 15 parts per hundred of rubber (phr), preferably between about 7 and 11 phr. Moreover, the present invention contemplates that carbon black will be added to mixture in a concentration between about 20 and 80 phr, preferably between about 50 and 70 phr.
While a uniform cross-section, with a thickness greater than 1 mm, for the fiber-reinforced elastomeric intermediate layer is contemplated, the present invention also recognizes that such layer may be manufactured with an uneven cross-section that may further improve the high speed test and handling characteristics of a tire so constructed. With such a construction, the present invention contemplates that the elastomeric intermediate layer, in cross-section, has thicker edges that together occupy about {fraction (4/7)}ths of the total width of the layer while the central portion occupies the remaining {fraction (3/7)}ths of the width. Moreover, the present invention contemplates that the edge portions are between about 25% to 75%, preferably about 33%, thicker than the central portion. Specifically, the present invention contemplates that the edge regions have a thickness greater than the central region of the elastomeric intermediate layer. The thicker portions of the elastomeric intermediate layer are better able to handle the high amounts of stress that develop at the edges of the tread band during rotation of the tire.
Additionally, the present invention contemplates that thicker regions may be added to the elastomeric intermediate layer in regions where the tread grooves are wider. The wider grooves in the tread act as deflection points where stresses can accumulate when the tire rotates. Adding thickness to the elastomeric intermediate layer in these regions helps to reduce the stresses during rotation of the tire.
The present invention also contemplates that the elastomeric intermediate layer will be coextruded with the tread band to create a uniform product. Coextrusion permits the correct orientation of the elastomeric intermediate layer to the tread band during construction. It also permits the correct orientation of the reinforcing fibers so that they lie substantially in the same equatorial direction as the 0xc2x0 nylon cords that they replace.